HID: amd_sfh: Handle condition of "no sensors"

Add a check for num_hid_devices to handle special case the situation
of "no sensors".

Fixes: 4b2c53d9

 ("SFH:Transport Driver to add support of AMD Sensor Fusion Hub (SFH)")
Signed-off-by: Basavaraj Natikar <Basavaraj.Natikar@amd.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>